In the prior art, organohydrogenpolysiloxanes are generally prepared by effecting equilibration reaction between a hydrosilyl-containing organosiloxane and another organosiloxane in the presence of an acidic catalyst, then neutralizing the catalyst, and recovering the end product via filtration and purification. Since a large amount of the acidic catalyst is necessary for equilibration reaction under acidic conditions, this process requires cumbersome treatment subsequent to the equilibration reaction. Specifically, the reaction solution must be washed with water to remove most of the acid and then treated with a neutralizing agent for complete removal of the acid. Also, when an organohydrogenpolysiloxane having alkoxy groups in a molecule is synthesized by this process, a problem arises that alkoxysilyl groups undergo hydrolysis during water washing. If neutralization is effected in a non-aqueous system to avoid hydrolysis, a greater amount of neutralizing agent is necessary, giving rise to problems including time-consuming removal of the neutralized salt and left-over of acid residues (e.g., sulfur, halogen, and phosphorus) in the organohydrogenpolysiloxane after neutralization because complete removal of such residues is difficult.
The organohydrogenpolysiloxane may then be reacted with an organic compound having an unsaturated group in the presence of a platinum catalyst according to a hydrosilylation reaction scheme:≡SiH+CH2═CH—→≡SiCH2—CH2—,thereby producing an organopolysiloxane having useful organic groups as disclosed in U.S. Pat. No. 5,472,987 (EP 0520392A1 or JP 2599237). Using this technique, an organopolysiloxane having both an alkoxy group and another useful organic group can be prepared.
One exemplary procedure starts with an organohydrogenpolysiloxane containing alkoxy groups represented by formula (A) and an organic compound having an unsaturated group represented by formula (B), thus producing an organopolysiloxane having both alkoxy groups and another organic group represented by formula (C).
Herein R is hydrogen or an arbitrary organic group, p and q are natural numbers.
Since this hydrosilylation reaction proceeds in the presence of a platinum catalyst, the conversion is reduced in the presence of catalyst poisons such as acid residues (e.g., sulfur, halogen, and phosphorus). In the preparation of organohydrogenpolysiloxane, it is thus essential to remove the entirety of the acidic catalyst after the completion of equilibration reaction.